A broadband radio frequency (RF) receiver may include a television tuner arranged to receive an RF television signal through an antenna or cable connection. The television tuner may implement multiple filters to attenuate signals which are outside of a frequency range of a desired frequency band or channel and to remove unwanted signals such as near adjacent interferers, far out interferers, harmonically related interferes, and so forth. In general, the filters attempt to remove the interfering signals before reaching active circuitry so as to effectively eliminate possible inter-modulation or blocking.
Conventional single conversion tuners may incorporate three or more tuned filter elements centered on a desired channel. Such filters may be configured as a first single tuned bandpass element and a second double tuned loosely coupled high quality factor (Q) bandpass arrangement. These filters typically are tuned by application of a varactor diode having a reverse bias capacitance inversely proportional to the applied voltage. In some cases, a television tuner may replicate such filters in three separate bands in order to cover a wide tuning range. In broadband tuners, for example, it is not physically possible to cover the 50 to 860 MHz tuning range in a single filter arrangement due to the large number of octaves tuned and the practical capacitance range available in commercial varactor diodes.
A module tuner may employ three cascaded tuned elements tuned to approximately the same frequency to provide a high Q response. Module tuners may comprise numerous discrete components including tuned filter elements fabricated from inductors (e.g., heavy gauge air coils) and low resistance varactor diodes. Such tuned filter elements are of high Q and provide a very high degree of attenuation to unwanted signals coupled with a minimum induced desired channel ripple. These filtering elements are physically aligned in production (e.g., adjusting wound inductive elements) to maximize performance, where performance is a balance between passband flatness, transition rate from passband to stopband and stopband attenuation.
When implementing a tuner in silicon, however, high Q inductors and varactor diodes are unavailable due to the number and value of components involved. In contrast to module tuners, for example, silicon tuners typically implement either a very simple tracking filter arrangement using a low Q single filter element or a phased feedback arrangement, whereby the feedback is arranged to provide a bandpass characteristic.
FIG. 1 illustrates a conventional silicon tuner 100 implementing tracking filters arranged as two parallel resonant LC bandpass elements indicated by L1//C1 and L2//C2. A simple switching mechanism including switches S1, S2 may be used to switch between frequency bands or channels. Each of these arrangements provides a bandpass response, which can be tuned by adjusting the capacitance value of each LC bandpass element to cover part of the RF frequency range. The limited Q of these circuits will limit the attenuation provided to undesired channels.